Gear shifts in an automatic transmission of a motor vehicle are usually initiated when a desired performance is called for by the vehicle's driver, for example, by acting on an accelerator pedal or throttle valve setting, exceeds an upshift or downshift characteristic of a shifting performance field of the automatic transmission. Besides these shifts initiated by way of the accelerator pedal, the driver can also initiate gear shifts manually at any time. Thus for example, DE 43 11 886 C2 shows a device which a driver can initiate gear shifts using a selector lever with a manual gate or switch-lever on the steering wheel.
In this, the gear shifts of the automatic transmission can for example be carried out as overlap shifts in which a first shift element opens and a second shift element closes. The pressure variation of the respective shift elements involved in the gear shift is usually controlled or regulated by an electronic transmission control unit, which acts upon a hydraulic shift element control system via electromagnetic control elements. Such a control method is known from DE 42 40 621 A1.
In practice, the following problem can now occur at the beginning of an intended overtaking process; the driver uses the accelerator pedal to call for a downshift which is then initiated by the transmission control unit. During the overtaking process just started, the driver then realizes that he must give way to oncoming traffic. As a reaction to this he interrupts the overtaking process by releasing the accelerator pedal. As a rule, this release of the pedal causes the transmission control to carry out an upshift. At a later time, still during the upshift, if the driver now sees that the opposite lane is free, he then resumes the overtaking process either by again actuating the accelerator pedal or by initiating a downshift manually using the selector lever. According to the prior art, however, the automatic transmission will first carry out the downshift completely and then the upshift and only then can another downshift be started.
As an improvement, a method for controlling an overlap gear shift is known from EP 0 341 631 B1 in which a first gear shift process that has been started is not completed if, after the passage of a blocking time after the shift command for the first gear shift, a shift command for a second gear shift is given within a time during which the clutch to be released is still transmitting torque and the clutch to be engaged is not yet transmitting any torque. If a second shift command is given after that moment, i.e., at the end of the first and during the second phase of the overlap gear shift, the first gear shift that has been started is first completed, a further blocking time is allowed to pass and only then is the next gear shift carried out.
To further increase the spontaneity of an electro-hydraulically controlled automatic transmission, a method is known from DE 197 22 954 C1 for the control of overlap gear shifts that take place in three phases, in which an upshift from a first to a second gear is, on the one hand, carried out immediately and, on the other hand, interrupted without delay, before the beginning of the third phase of the overlap upshift in progress, if a driver-specific command for a downshift is recognized. In this, the first phase of the overlap gear shift is defined as the filling and control phase during which the shift element to be engaged is being filled with pressure medium and a speed increase of a transmission input speed is adjusted by a pressure reduction of the shift element being disengaged. The second phase corresponds to the load uptake by the shift element being engaged and begins with a reduction of the speed increase. In the third phase, which begins when the synchronization point of the first (old) gear is reached again, the shift element being engaged is closed completely and the transmission input speed is brought to the level of the synchronous speed of the second (new) gear.
From EP 0 800 022 B1, a method for controlling a gear shift process is known in which an upshift from a first to a second gear initiated by a first shift command is interrupted and the first gear returned to, when, during the closing phase of the upshift, in which a transmission input speed is changed from the synchronous speed of the first gear to the synchronous speed of the second gear by the shift pressure of the shift element being closed, a second shift command for a downshift is recognized. To those with knowledge of the field it is clear that this method can only be used properly in combination with automatically initiated free-running for automatic torque uptake during the return to the original first gear. Its use with an overlap gear shift, especially in the case of an upshift interruption at a late stage of the upshift closing phase, leads to considerable comfort problems (shift jerks) due to the clutch overlap.
Thus, on one hand, the problem is that there can be a large time lag between the driver's wish concerning the acceleration of the motor vehicle and the reaction of the automatic transmission, particularly in combination with overlap gear shifts, especially when a downshift-upshift-downshift sequence is called for. On the other hand, because of continual shifting, the shifting behavior of the automatic transmission can be perceived by the driver as hectic.
The purpose of the present invention is to further improve the spontaneity of a motor vehicle automatic transmission without adverse effect on the quality of shifting, in particular with overlap gear shifts and to reduce the shifting frequency of the automatic transmission perceptible by the driver.
According to the invention, this objective is achieved by the characteristics of claim 1 and by a method having the characteristics of the auxiliary claims 2, 4 or 6. Advantageous features of the invention emerge from the subordinate claims.